TMCI risk from special components (collimators) in STCF

Determine whether installing special components, particularly beam collimators, in the Super Tau-Charm Facility (STCF) storage ring will induce transverse mode coupling instability (TMCI) at the design single-bunch current of approximately 3.9 mA, given that calculations for the ordinary vacuum chamber (without such components) estimate a transverse effective impedance of about 14 kΩ/m and a TMCI threshold current of roughly 28.9 mA.

Background

In the STCF design, impedance-induced collective effects are critical for beam stability. Using an estimated longitudinal normalized effective impedance Im(Z∥/n)eff ≈ 0.1 Ω, the authors infer an associated transverse effective impedance of approximately 14 kΩ/m via the Panofsky–Wenzel relation, yielding a TMCI threshold of about 28.9 mA, which is well above the design single-bunch current of roughly 3.9 mA.

While ordinary vacuum chamber impedance is not expected to cause TMCI under these assumptions, the inclusion of special components—especially beam collimators—can significantly alter the ring impedance. The authors explicitly state that it remains to be determined whether such additions could induce TMCI, highlighting an unresolved stability risk that must be assessed in the detailed machine design.